Method of manufacturing twinning induced plasticity type ultra-high strength steel sheet

ABSTRACT

The present invention features a method of manufacturing a TWIP type ultra-high strength steel sheet, which can improve the yield strength, tensile strength and elongation rate of the TWIP type ultra-high strength steel sheet by appropriately adjusting the amounts of carbon (C), silicon (Si), manganese (Mn), aluminum (Al), molybdenum (Mo), phosphorus (P) and sulfur (S).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to KoreanApplication No. 10-2008-0087282, filed on Sep. 4, 2008, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, generally, to a method of manufacturing atwinning induced plasticity (TWIP) type ultra-high strength steel sheetand, more particularly, to a method of manufacturing a TWIP typeultra-high strength steel sheet for vehicle body components, which cansuitably increase yield strength, tensile strength and elongation rate.

2. Description of the Related Art

Generally, ultra-high strength steel sheets which are widely used asmaterials for automotive body components have a tensile strength of590˜780 MPa, a yield strength of 270˜350 MPa, an elongation rate of25˜35% and a plastic strain ratio of 0.9˜1.2.

However, when applying those ultra-high strength steel to automotivebody components, cracks, corrugating, and the like, can be caused by aninsufficient elongation rate at the time of press forming, which may beproblematic. Thus a thick steel sheet is used in consideration of thestrength of vehicle body components. Further, even though elongation issufficiently ensured, it is generally difficult to form a steel sheetinto vehicle body components because the vehicle body components arecomplicated and multi-functionalized. Therefore, the plastic strainratio of a steel sheet is preferably required to be considerablyincreased with the development of forming technologies.

Korean Unexamined Patent Application Publication No. 2007-0018416,incorporated by reference in its entirety herein, discloses a twinninginduced plasticity type ultra-high strength steel sheet, comprising:0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % ofmanganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus,0.001˜0.002 wt % of sulfur, and residual iron and other inevitableimpurities.

Although the above twinning induced plasticity (TWIP) type ultra-highstrength steel sheet has remarkable material properties, it is alsoincreasingly required to have high collision strength and to be used forcomplicated vehicle body components. Thus, it is an object of theinvention to improve the yield strength, tensile strength and elongationrate of the TWIP type ultra-high strength steel sheet together. Thereason for this is because the defective fraction in the formation of aproduct is suitably increased when its elongation rate is low.

Accordingly, various alloy elements are required to be added to thesteel sheet. However, the use of the alloy elements may be suitablylimited due to the rise in the price of raw materials and due to therequirement of the use of environment-friendly materials. Therefore,there is a need in the art for the development of methods ofconsiderably improving the material properties of a steel sheet withoutchanging the composition of the steel sheet.

The above information disclosed in the Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of manufacturinga TWIP type ultra-high strength steel sheet, which can suitably increasethe yield strength, tensile strength and elongation rate of the TWIPtype ultra-high strength steel sheet.

In preferred embodiments, the present invention provides a method ofmanufacturing a TWIP type ultra-high strength steel sheet, comprising:cold-rolling a hot-rolled steel sheet having a composition including0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15˜25 wt %of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less ofphosphorus (P), 0.001˜0.002 wt % of sulfur (S), and residual iron (Fe)and other inevitable impurities in four passes or more; suitablyrecovering the cold-rolled steel sheet at a temperature of 200˜220° C.after the third pass of the cold rolling; and annealing the recoveredsteel sheet.

In further preferred embodiments of the method, the recovering of thecold-rolled steel sheet may preferably be conducted for 5˜6 minutes.

In related embodiments, the annealing of the recovered steel sheet maypreferably be conducted at a temperature of 700˜850° C. for 5˜6 minutes.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum).

As referred to herein, a hybrid vehicle is a vehicle that has two ormore sources of power, for example both gasoline-powered andelectric-powered.

The above features and advantages of the present invention will beapparent from or are set forth in more detail in the accompanyingdrawings, which are incorporated in and form a part of thisspecification, and the following Detailed Description, which togetherserve to explain by way of example the principles of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a graph showing the grain size of a TWIP type ultra-highstrength steel sheet depending on annealing temperature according topreferred embodiments of the invention;

FIG. 2 is a graph showing the yield strength of a TWIP type ultra-highstrength steel sheet depending on annealing time according to Examplesof the present invention;

FIG. 3 is a graph showing the elongation rate of a TWIP type ultra-highstrength steel sheet depending on annealing time according to Examplesof the present invention; and

FIG. 4 is a graph showing the yield strength of a TWIP type ultra-highstrength steel sheet depending on the elongation rate thereof accordingto the Examples of the present invention and the Comparative Examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first aspect, the invention features a method of manufacturing aTWIP type ultra-high strength steel sheet, comprising cold-rolling ahot-rolled steel sheet having a composition including 0.15˜0.30 wt % ofcarbon (C), 0.01˜0.03 wt % of silicon (Si), 15˜25 wt % of manganese(Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less of phosphorus(P), 0.001˜0.002 wt % of sulfur (S), and residual iron (Fe) and otherinevitable impurities in four passes or more; and recovering thecold-rolled steel sheet.

In one embodiment, the cold-rolled steel sheet is recovered at atemperature of 200˜220° C.

In another embodiment, the cold-rolled steel sheet is recovered afterthe third pass of the cold rolling.

In still another embodiment, the recovering of the cold-rolled steelsheet is conducted for 5˜6 minutes.

In another particular embodiment, the method further comprises annealingthe recovered steel sheet.

In one embodiment, the annealing of the recovered steel sheet isconducted at a temperature of 700˜850° C.

In a further embodiment, the annealing of the recovered steel sheet isconducted for 5˜6 minutes.

Hereinafter, a method of manufacturing a TWIP type ultra-high strengthsteel sheet according to preferred embodiments of the present inventionwill be described in detail with reference to the attached drawings.

According to certain preferred embodiments of the invention, the TWIPtype ultra-high strength steel sheet has a composition preferablyincluding 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si),15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt %or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), and residualiron (Fe) and other inevitable impurities.

According to certain preferred embodiments of the invention, the methodof manufacturing a TWIP type ultra-high strength steel sheet accordingto a preferred embodiment of the present invention is suitably the sameas a conventional method concerning the steps of suitably melting thesteel sheet composition in a converter, suitably continuous-casting themolten steel sheet composition to form a steel sheet, suitablyhot-rolling the steel sheet at a temperature of 1100˜1300° C. andsuitably winding the hot-rolled steel sheet.

Preferably, after the winding of the hot-rolled steel sheet, the hotrolled steel sheet is suitably cold-rolled, preferably in five passes,and then the cold-rolled steel sheet is suitably recovered at atemperature of 200˜220° C. after the third pass of the cold rolling. Infurther embodiments, the recovered steel sheet is suitably annealed at atemperature of 700˜850° C. In other preferred embodiments, therecovering of the cold-rolled steel sheet may be conducted for 5˜6minutes, and the annealing of the recovered steel sheet may be conductedat a temperature of 700˜850° C. for 5˜6 minutes.

Preferably, recovering of the cold-rolled steel sheet is conducted is tosuitably accelerate the generation of subgrains in the grains of anaustenite matrix by inducing the combinations between dislocations andtwins. In more particular embodiments, the combinations betweendislocations and twins are suitably induced through the recovering ofthe cold-rolled steel sheet after the third pass of the cold rolling,and the combined dislocation and twins are suitably formed intosubgrains in the grains of an austenite matrix through the forth andfifth passes of the cold rolling.

According to further preferred embodiments of the invention, the coldrolling having five passes may preferably be conducted at a rollingreduction ratio of 20˜30% per pass in a similar manner to conventionalcold rolling having 5˜7 passes which is preferably conducted at arolling reduction ratio of about 30% per pass, and is generally used tomanufacture high-strength steel sheets as well as TWIP type ultra-highstrength steel sheets.

As described herein, it was found that a recovery process must besuitably performed during the cold rolling process because twinning andslipping simultaneously occur in the deformation mechanism of a TWIPtype ultra-high strength steel sheet, unlike in the case of a generalsteel sheet. Accordingly, when 5 passes of cold rolling are made, therecovery process is suitably performed after the third pass of the coldrolling.

Preferably, when the recovery process is suitably performed after thefirst or second pass of the cold rolling, the expected results cannot besuitably achieved due to the ungrown subgrains. According to furtherpreferred embodiments of the invention, when the recovery process isperformed after the fourth pass of the cold rolling, desired materialproperties cannot be suitably obtained at the time of annealing becausea low angle boundary is formed due to the misorientation between grownsubgrains. Accordingly, it is preferred that the recovery process besuitably performed after the third pass of the cold rolling.

According to further preferred embodiments, the annealing of therecovered steel sheet is preferably conducted at a temperature of700˜850° C. for a short period of time (5˜6 minutes) in order tosuitably decrease the grain size of the TWIP type ultra-high strengthsteel sheet to 2˜3 μm. Accordingly, the elongation rate thereof can besuitably increased by decreasing the grain size thereof.

Hereinafter, preferred embodiments of the present invention will bedescribed in more detail with reference to the following Examples andComparative Examples.

In the Examples and Comparative Examples described herein, a TWIP typeultra-high strength steel sheet is manufactured according to certainpreferred embodiments of the present invention using the compositiongiven in Table 1, and then the mechanical properties thereof weresuitably measured through tension testing, and the grain size thereofwas suitably analyzed through electron back scattered diffraction(EBSD).

TABLE 1 C Si Mn Al P S Fe Chemical 0.15~0.30 0.01~0.03 15.0~25.01.20~3.00 0.020 0.001~0.002 residual components or less (wt %)

The results obtained from the measurement and analysis are given inTable 2 and Table 3. In particular preferred embodiments of theinvention, for example as shown in the Examples, a slab, which had beenprepared by suitably melting the composition in a converter and thencontinuous-casting the molten composition, was hot-rolled from 1300° C.to 1100° C., was cooled from 900° C. to 600° C. preferably at a coolingrate of 40° C./sec and then winded, was cold-rolled, preferably throughfive passes at a rolling reduction ratio of 30% or less per pass duringwhich the slab was preferably heat-treated at 200˜220° C. for 5 minutesafter the third pass of cold rolling and then the residual two passesthereof were preferably performed, and was then suitably annealed at700˜850° C. for 5 minutes using a continuous annealing furnace, therebysuitably decreasing the grain size thereof.

TABLE 2 Recovery Recovery Annealing Annealing Yield Tensile Averagetemperature time Recovery temperature time strength strength Elongationgrain (° C.) (min) pass (° C.) (min) (MPa) (MPa) rate (%) size (μm) Ex.1 200 5 3 700 5 580 1020 53 2.1 Ex. 2 200 5 3 750 5 580 1020 53.2 2.3Ex. 3 200 5 3 800 5 560 992 52.1 2.5 Ex. 4 200 5 3 850 5 520 989 52.12.9 Ex. 5 220 5 3 700 5 592 1008 52.3 2.0 Ex. 6 220 5 3 750 5 590 101052.2 2.12 Ex. 7 220 5 3 800 5 577 998 52.8 2.6 Ex. 8 220 5 3 850 5 580992 53.1 2.88

According to further embodiments of the invention as described herein,the Comparative Examples are similar to or the same as Examples, exceptthat the cold rolling was preferably performed at a rolling reductionratio of 30% or less per pass through five passes and then the annealingwas preferably performed at 850° C. for 8˜10 hours using a box furnace.

TABLE 3 Recovery Recovery Annealing Annealing Yield Tensile ElongationAverage temperature time Recovery temperature time strength strengthrate grain (° C.) (min) pass (° C.) (min) (MPa) (MPa) (%) size (μm)Comp. Ex. 1 — — — 850 480 510 978 48.2 6.83 Comp. Ex. 2 — — — 850 540502 978 48.5 9.35 Comp. Ex. 3 — — — 850 600 490 950 48.8 12.1 Comp. Ex.4 200 5 3 850 480 505 980 48 7.0 Comp. Ex. 5 200 5 3 850 540 493 96048.2 11.1 Comp. Ex. 6 200 5 3 800 600 462 963 48.5 12.4 Comp. Ex. 7 2205 3 850 480 499 942 46.5 8.3 Comp. Ex. 8 220 5 3 850 540 493 931 47.39.2 Comp. Ex. 9 220 5 3 800 600 460 922 48.1 12.4 Comp. Ex. 200 4 3 7005 530 980 42.1 3.3 10 Comp. Ex. 200 4 3 850 5 510 977 43.2 4.2 11 Comp.Ex. 220 4 3 700 5 523 977 42.8 3.5 12 Comp. Ex. 220 4 3 850 5 499 96344.6 3.9 13 Comp. Ex. 200 7 3 700 5 510 977 41.2 3.8 14 Comp. Ex. 200 73 850 5 503 973 40.2 4.1 15 Comp. Ex. 220 7 3 700 5 511 974 45.1 3.9 16Comp. Ex. 220 7 3 850 5 482 958 42.6 4.7 17

According to still further embodiments of the invention, in order tosuitably determine the annealing temperatures of the Examples, the TWIPtype ultra-high strength steel sheet was preferably heat-treated from600° C. to 920° C. for 5 minutes, and then the grain size thereof wasmeasured. The results thereof according to certain preferred embodimentsof the invention as described herein are shown in FIG. 1. Referring toFIG. 1, it can be seen that the grain size thereof at a temperaturerange of 700˜850° C. is about 2˜3 μm.

According to further embodiments, it was found that the TWIP typeultra-high strength steel sheet was not suitably recrystallized at atemperature of less than 700° C., and that, according to other furtherembodiments, the elongation rate of a final product did not reach 20%.Accordingly, the annealing was not performed at a temperature of lessthan 700° C..

In other further embodiments of the invention and referring to theresults given in Table 2 and Table 3, it was found that the yieldstrengths of the Examples of the present invention were suitablyincreased by 30 MPa˜100 MPa compared to those of Comparative Examples 1to 3, and that the elongation rates of the Examples of the presentinvention were also suitably increased by 3˜4% compared to those ofComparative Examples 1 to 3. Generally, according to certain preferredembodiments, elongation rate is suitably decreased with an increase instrength. However, in the case of certain preferred Examples of thepresent invention as described herein, both strength and elongation ratewere suitably increased due to the twins existing in subgrains generatedthrough the recovery process.

From Comparative Examples 4 to 9, it can be seen that according tofurther embodiments of the invention, there is no effect when theannealing conditions are suitably the same as conventional annealingconditions even though the recovery process during the cold rollingprocess is preferably conducted the same as in the Examples. In furtherpreferred embodiments, and from Comparative Examples 10 to 17, it can beseen that it is most effective when the recovery time during the coldrolling process is 5 minutes.

According to still other embodiments of the present invention, thereason why the annealing time of the Examples is preferably set to 5minutes is that, when the annealing time is less than 5 minutes, theTWIP type ultra-high strength steel sheet is suitably slightlyrecrystallized, and thus the increase in the elongation rate thereofcannot be expected.

In other particular embodiments, when the annealing time is above 5minutes or excessively above 5 minutes, the increase in the elongationrate of the TWIP type ultra-high steel sheet can be expected, but thestrength thereof is rapidly decreased due to the overgrowth of grains.According to exemplary embodiments of the invention, and as shown inFIG. 2, a reason for this can be verified from FIG. 2 showing thesuitable decrease in the yield strength of the TWIP type ultra-highstrength steel sheet preferably depending on annealing time at 700° C..Accordingly, in certain embodiments of the invention, it is preferredthat the annealing be conducted for 5˜6 minutes, more preferably 5minutes. Accordingly, in other exemplary embodiments of the inventionand as shown in FIG. 3, a reason for this can also be verified from FIG.3 showing the change in the elongation of the TWIP type ultra-highstrength steel sheet preferably depending on annealing time.Accordingly, referring to FIG. 3, it can be seen that a preferredelongation rate of 50% or more preferably can be obtained when theannealing time is 5˜6 minutes.

TABLE 4 Inter. Inter. Inter. Average heat heat heat Annealing AnnealingYield Tensile Elongation grain treatment treatment treatment temperaturetime strength strength rate size temp. (° C.) time (min) pass (° C.)(min) (MPa) (MPa) (%) (μm) Ex. 1 200 5 3 700 5 580 1020 53 2.1 Ex. 4 2005 3 850 5 520 989 52.1 2.9 Ex. 5 220 5 3 700 5 592 1008 52.3 2.0 Ex. 7220 5 3 800 5 577 998 52.8 2.6 Comp. 200 5 4 700 5 492 977 46.3 4.1 Ex.18 Comp. 200 5 4 850 5 488 976 46.3 3.9 Ex. 19 Comp. 220 5 4 700 5 479943 45.5 4.0 Ex. 20 Comp. 220 5 4 800 5 482 930 46.1 4.6 Ex. 21 Comp.200 5 2 700 5 490 975 47.1 3.7 Ex. 22 Comp. 200 5 2 850 5 490 975 47.33.6 Ex. 23 Comp. 220 5 2 700 5 483 950 43.5 4.2 Ex. 24 Comp. 220 5 2 8005 480 945 46.2 3.9 Ex. 25

In other embodiments of the invention, in order to suitably verify thechange in the material properties of the TWIP type ultra-high strengthsteel sheet at the time of recovering the TWIP type ultra-high strengthsteel sheet manufactured using the composition given in Table 1 duringthe cold rolling process, the material properties of the TWIP typeultra-high strength steel sheet of Examples 1, 4, 5 and 7 andComparative Examples 18 to 25 are given in Table 4. In furtherembodiments, and referring to Table 4, from Comparative Examples 18 to25, it can be seen that the material properties of the TWIP typeultra-high strength steel sheet are not influenced by the recoveryprocess suitably performed after the fourth pass or second pass of thecold rolling process. Therefore, according to further preferredembodiments of the invention, it is preferred that the recovery processpreferably be performed after the third pass of the cold rollingprocess.

According to the above described methods of manufacturing the TWIP typeultra-high strength steel sheet according to preferred embodiments ofthe present invention, the yield strength of the TWIP type ultra-highstrength steel sheet can be suitably increased by a maximum of 100 MPacompared to that of conventional steel sheets, the elongation ratethereof can be suitably increased by 3˜4% compared to that ofconventional steel sheets to obtain an elongation rate of 50% or more,and the tensile strength thereof can also be suitably increased to 980MPa. Preferably, a TWIP type ultra-high strength steel sheet, which hassuitably high collision strength and can be suitably formed intocomplicated vehicle body components, can be suitably manufactured.Accordingly, preferred embodiments of the present invention are show inFIG. 4, where FIG. 4 shows the yields strengths and elongation rates ofthe Examples and the Comparative Examples. Referring to FIG. 4, fromdata A, it can be seen that in certain exemplary embodiments, the yieldstrengths of the Examples are 520˜592 MPa and the elongation ratesthereof are 50% or more. In other embodiments, from data B, it can beseen that the yield strengths of the Comparative Examples are 520 MPa orless and the elongation rates thereof are 50% or less.

As described herein, preferred methods of manufacturing a TWIP typeultra-high strength steel sheet according to the preferred embodimentsof present invention are advantageous in that the yield strength,tensile strength and elongation rate of the TWIP type ultra-highstrength steel sheet can be simultaneously improved, and thus thedefective fraction in the formation of a vehicle body component can bedecreased.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of manufacturing a TWIP type ultra-high strength steelsheet, comprising: cold-rolling a hot-rolled steel sheet having acomposition including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % ofsilicon (Si), 15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum(Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur(S), and residual iron (Fe) and other inevitable impurities in fourpasses or more; recovering the cold-rolled steel sheet at a temperatureof 200˜220° C. after the third pass of the cold rolling; and annealingthe recovered steel sheet.
 2. The method of manufacturing a TWIP typeultra-high strength steel sheet according to claim 1, wherein therecovering of the cold-rolled steel sheet is conducted for 5˜6 minutes.3. The method of manufacturing a TWIP type ultra-high strength steelsheet according to claim 1, wherein the annealing of the recovered steelsheet is conducted at a temperature of 700˜850° C. for 5˜6 minutes.
 4. Amethod of manufacturing a TWIP type ultra-high strength steel sheet,comprising: cold-rolling a hot-rolled steel sheet having a compositionincluding 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si),15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt %or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), and residualiron (Fe) and other inevitable impurities in four passes or more; andrecovering the cold-rolled steel sheet.
 5. The method of manufacturing aTWIP type ultra-high strength steel sheet of claim 4, wherein thecold-rolled steel sheet is recovered at a temperature of 200˜220° C.. 6.The method of manufacturing a TWIP type ultra-high strength steel sheetof claim 5, wherein the cold-rolled steel sheet is recovered after thethird pass of the cold rolling.
 7. The method of manufacturing a TWIPtype ultra-high strength steel sheet according to claim 6, wherein therecovering of the cold-rolled steel sheet is conducted for 5˜6 minutes.8. The method of manufacturing a TWIP type ultra-high strength steelsheet of claim 4, further comprising annealing the recovered steelsheet.
 9. The method of manufacturing a TWIP type ultra-high strengthsteel sheet according to claim 8, wherein the annealing of the recoveredsteel sheet is conducted at a temperature of 700˜850° C..
 10. The methodof manufacturing a TWIP type ultra-high strength steel sheet accordingto claim 8, wherein the annealing of the recovered steel sheet isconducted for 5˜6 minutes.